1. Field of the Invention
This invention generally relates to redundant data storage devices in a data processing network and more particularly to a method and apparatus that enables data to cascade through multiple redundant data storage units.
2. Description of Related Art
The maintenance of data integrity by data redundancy has become a very important issue. Data redundancy has several forms or variations. At a single site, mirroring or RAID redundancy protects against disk or other storage failure. In another form described in U.S. Pat. No. 6,092,066 and others, redundancy is achieved by duplicating a local system at a remote location. The use of a remote location prevents data loss due to natural disasters and the like at one site.
In accordance with the foregoing U.S. Pat. No. 6,092,066 all data processing activity occurs at a xe2x80x9clocalxe2x80x9d or xe2x80x9cproductionxe2x80x9d site that contains a host system for processing data stored in a data storage facility. A geographically remote or backup site includes a data storage facility as a xe2x80x9credundantxe2x80x9d facility for maintaining a restoration or recovery data set. In this system each time the host at the production site writes data to the production site data storage facility, the production data storage facility automatically writes data to the remote storage facility. In many, if not most, of these applications, writing data to the remote site data storage facility requires a transfer across a high bandwidth communications link so the backup procedure does not affect operations at the production facility. T3 and ESCON lines are typically preferred as the communications links despite their expense.
U.S. Pat. No. 6,101,497 Ser. No. 08/842,953 discloses another concept involving a BCV device typically used at the production site. In this approach a data set, for example a xe2x80x9clogical volumexe2x80x9d, on a production site data storage facility, i.e., a xe2x80x9cproduction volumexe2x80x9d, operates in a normal fashion. Another logical volume at this same site is designated as a xe2x80x9cBCV volumexe2x80x9d. An ESTABLISH command connects the BCV volume to the production volume so that the BCV volume synchronizes with the production volume. A SPLIT command thereafter can separate the BCV volume from the production volume making the data stored on the BCV volume available for another application.
This other application may alter the data stored on the BCV volume. Consequently whenever the BCV volume is reconnected with the production volume, it is necessary to transfer data to the BCV volume that represent changes in both the BCV volume and the production volume. An alternative is to reconnect the BCV volume to the production volume by issuing another ESTABLISH command. However, this command will replace all the data on the BCV volume so all the data from the production volume must be transferred to the BCV volume whether or not changes have occurred.
In the past locating data storage facilities at a production site and a single remote site has been acceptable. In certain critical applications it is now also desirable, and in some situations mandatory, to store still another restoration copy at a third site that is remote from the first two sites. This requires some approach for copying the data from the production site to both remote sites, all transparently to the operations at the production site. Merely repeating the foregoing approach for copying data from a production site to a single remote site involves excessive communications costs. A second high-speed communications link will be required between either the production site and the second remote site or between the first and second remote sites. What is needed is a way to establish a redundant data copy over a less costly communications link without any significant disparity in the data that exists at various remotely located sites such that all the data storage facilities are in synchronism or nearly in synchronism.
Therefore it is an object of this invention to provide a method and apparatus for establishing redundant data storage facilities that can communicate over less costly communications links.
Another object of this invention is to provide a method and apparatus for achieving redundant disk storage at geographically remote sites on an economical basis.
Still another object of this invention is to provide a method and apparatus for providing redundant data storage at two or more geographically remote sites using an economical communications link.
Yet another object of this invention is to provide a method and apparatus for providing redundant data storage at two or more geographically remote sites using an economical low-bandwidth communications link.
Yet still another object of this invention is to provide a data processing network in which data processed at a production site is stored at two or more redundant storage sites with redundancy being achieved transparently to operations at the production site.
In accordance with this invention, a data storage site remote from a data processing facility that alters data, such as a production facility, transfers data to another site remote from the data storage facility. The data storage facility at the remote site includes first and second data stores. The first data store receives data from the production site. A data change recorder identifies changes that the data processing facility makes in the first data store. A first operating control establishes a first operating mode during which the second data store receives data from the first data store according to the changes recorded in said change recording means. After this operation is complete, a second operating control can establish a second operating mode for copying data from the second data store to the other remote site according to the changes recorded in said data change recorder.
In accordance with another aspect of this invention, data is transferred between a production site and a remote site. The production site includes a host and production storage facility; the remote site, a remote storage facility including a first data store and a second data store wherein the first data store receives data from the production facility on a track-by-track basis. The host can issue a plurality of cascade commands to enable the definition of a plurality of track status tables for identifying each track in the first data store that the production facility changes. In addition, the host can establish first and second operating modes. In the first operating mode the second data store receives data from the first data store according to the changes recorded in the track status tables. During the second operating mode data from the second data store is copied to the remote storage facility according to the changes recorded in the track status tables at the time the second mode is established.